1. Field of the Invention
The present invention relates to a wet-type multi-plate friction engaging apparatus in a clutch or a brake of an automatic transmission of a motor vehicle, a lock-up clutch, a starting clutch and the like.
2. Related Background Art
A wet-type multi-plate clutch i.e. wet-type multi-plate friction engaging apparatus comprising paper friction materials has various advantages that torque to be transmitted can be controlled on the basis of a load applied to a friction surface and that permits smooth engagement during the transmission of torque and is used mainly and widely in speed changing devices of automatic transmissions, torque converters, starting clutches or the like. One example of such starting clutches is shown in Japanese Patent Application Laid-Open No. 2006-105397.
Excellent heat resistance and smooth engagement is required for the wet-type multi-plate clutch used in the automatic transmission.
A conventional wet-type multi-plate friction engaging apparatus is constituted as shown in FIG. 12. The conventional wet-type multi-plate friction engaging apparatus 100 comprises internally toothed plates 105 each of which is obtained by sticking paper friction materials 104 onto both surfaces of a metal plate, externally toothed plates 106 formed from metal plates, the plates 105 and 106 being arranged alternately within a clutch drum 101, a piston 107 for urging the internally toothed plates 105 and the externally toothed plates 106 against each other, a backing plate 103, and a snap ring 102 for supporting the internally toothed plates 105, externally toothed plates 106 and backing plate 103.
In the above-mentioned conventional apparatus, since camber deformation of each plate and warping deformation of the clutch drum 101 are generated by the urging of the piston 107, there is a tendency that face pressure on the friction surface becomes greater at an outer diameter side than at an inner diameter side toward the opening end of the clutch drum 101. Such a tendency becomes more noticeable as rigidity of the clutch drum 101 is decreased and as a thickness of the backing plate 104 is decreased, and, in particular, the outer diameter side face pressure on the friction material 104 at the opening end of the clutch drum 101 becomes greater. As a result, since the whole surface of each friction plate 104 does not work effectively, heat resistance may be reduced and smooth movements of the plates may be prevented, thereby affecting a bad influence upon the engaging property.
Further, in recent years, as light weight, increase in stages of the automatic transmission and reduction of cost have been requested, weights of the clutch drum and the backing plate are requested to be reduced and, at the same time, burning or baking of the clutch has been increased because of rotation at high speed.